Abrasives are used in a variety of settings to produce a desired surface finish on a workpiece. Within the field of microfinishing, abrasives are used to abrade specified amounts of material from a workpiece to provide a surface finish that meets certain parameters. In the automotive field, for example, journals such as camshafts and crankshafts for internal combustion engines must meet exacting standards for geometry and surface finish. If a camshaft or a crankshaft is improperly sized or finished, uneven wear patterns may result, and could lead to failure of that component or other components within the engine.
The present invention relates primarily to abrading an outer peripheral surface of a workpiece, such as the bearing surfaces of the journal, shown as a camshaft in FIG. 1. One manner of microfinishing such a surface is to provide a support shoe having a pressure face against which an abrasive sheet or tape is placed, contact the abrasive face of the tape to the peripheral surface, and rotate the workpiece with respect to the support shoe. The abrasive tape may be, for example, a coated abrasive, a lapping abrasive, or a nonwoven abrasive. Preferred abrasive products for these applications are fine grade abrasive grains that range in average particle size from less than 0.1 up to 200 micrometers, preferably between about 5 to 125 micrometers. The support shoe can be made out of any material that is sufficiently durable to withstand the rigors of the abrading process. Common materials for the pressure face include but are not limited to urethanes, India stone materials, metals or hard coatings on metals. The pressure face may be unitary, or may include multiple pressure face segments that combine to form a profile that matches that of an outer peripheral surface of a workpiece.
FIGS. 1 and 2 illustrate an apparatus 10 for abrading material from the individual peripheral surfaces of a workpiece 12. The support shoes 14 and 16 include pressure faces 18 and 20 that are typically concave, and match the desired profile of the peripheral surface of the workpiece 12 being abraded. In the illustrated embodiment, two semicylindrical pressure faces 18 and 20 large abrasive tape 22 against surface 24 of workpiece 12. When workpiece 12 is rotated, abrasive tape 22 abrades material from the outer peripheral surface of workpiece 12, due to pressure from pressure faces 18 and 20 against the surface. Pressure faces 18 and 20 may also be moved transversely across the peripheral surface of workpiece 12 as the workpiece is rotated, as shown by directional arrows 15. Transverse motion of the pressure faces produces a multidirectional scratch pattern on the surface of the workpiece, which may be desirable for certain applications. In the case of microfinishing a camshaft or a crankshaft (i.e. abrading minute amounts of material from a surface), more than one peripheral surface may be abraded simultaneously. Camshaft and crankshaft microfinishing is described in U.S. Pat. Nos. 4,682,444 (Judge et al.) and 4,993,191 (Judge et al.).
For some applications, lubricants such as mineral seal oil are provided at the abrasive interface between the surface of the workpiece and the abrasive tape to carry abraded particles away from the abrasive interface, and to enable increased heat transfer away from the workpiece. These lubricants are preferably water soluble to facilitate cleaning of the work area. However, because the abrasive tape is subjected to a rotary shear force during abrading, and to a shear force if the workpiece is moved transversely (as shown by directional arrow 15 in FIG. 1), the lubricant tends to facilitate slippage between the abrasive tape and the pressure face. It is important to maintain the abrasive tape in position with respect to the pressure face, and thus slippage is undesirable because the abrasive tape may become displaced with respect to the pressure face.
Moreover, given a sufficient amount of displacement, the abrasive tape may not be properly located over the pressure face, causing uncontrolled scratches in the surface of the workpiece and potentially dislodging or tearing the abrasive tape. Furthermore, because the abrading process may be automated, a dislocation of or break in the tape may damage not only the workpiece currently being abraded, but several or even dozens of successive workpieces before the disruption is discovered. If the abrasive tape has been broken, it may wrap around the workpiece, which may in turn cause the manufacturing line to shut down, which is time consuming and undesirable. If the abrasive tape breaks, the entire production line may have to be halted, so that the abrasive tape may be threaded through the abrading apparatus again, which is a costly and therefore undesirable procedure.
One manner of reducing slippage of the abrasive tape with respect to the pressure face beneath the tape is to apply a slip resistant coating to the back face of the abrasive tape.. For example, Minnesota Mining and Manufacturing Company of St. Paul, Minn. sells a 262L or 272L Imperial Microfinishing film product Type S, and a 263L or 273L Imperial Microfinishing film product Type Q. Each film includes a slip resistant coating disposed on the back face of the film, comprising an inorganic particulate dispersed in a polymeric binder. The slip resistant coating tends to reduce slippage between the abrasive tape and the pressure face, resulting in more satisfactory abrading processes than those described above.
Although slip resistant coatings may alleviate some slippage of the abrasive tape, other problems may render the use of slip resistant coatings undesirable. For example, it is possible for the slip resistant coating such as an adhesive to transfer to and subsequently build up on the support shoe, which may cause the abrasive tape to abrade unevenly. Even small deposits of a slip resistant coating can raise the effective height of the support shoe, and can result in excessive abrading of the workpiece. In an automated environment, the accumulation of small amounts of slip resistant coating over a period of time may therefore result in workpieces being microfinished to different sizes. This may represent a sacrifice of consistency and accuracy in microfinishing in exchange for the slip resistant properties of the coating, which is unacceptable.
It is therefore desirable to provide a method and apparatus for releasably positioning an abrasive tape on a support shoe for abrading a workpiece, and to reduce slippage between the abrasive tape and the support shoe during abrading, without using slip resistant coatings.
Abrasive sheets and tapes have a certain useful life, after which they begin to degrade, causing irregular microfinishing of the workpiece. It is therefore desirable to advance the abrasive tape periodically, to provide a new abrasive surface for application to the workpiece. Advancing the abrasive tape is known in the art as "indexing" the abrasive tape, and the tape is typically indexed between 1/8" and 8", and more typically between 1/2" and 1" after a particular surface has been finished. Thus, it is therefore desirable to provide a method and apparatus for abrading a workpiece, wherein the abrasive tape may be indexed periodically.